In multicast and broadcast applications, data are transmitted from a server to multiple receivers over wired and/or wireless networks. A multicast system as used herein is a system in which a server transmits the same data to multiple receivers simultaneously, where the receivers form a subset of all the receivers up to and including all of the receivers. Instead of making a copy of the data for each receiver, in multicast a single copy of the data is transmitted. A broadcast system is a system in which a server transmits the same data to all of the receivers simultaneously. That is, a multicast system by definition can include a broadcast system.
Multicast is, however, unable to recover packet errors. This problem is more acute in certain types of networks (e.g. wireless networks). In such cases unicasting is better suited for data transmission as most of the link layer protocols use retransmissions to recover errors in data packet transmission. An example scenario would be an IPTV service, which may multicast multimedia (video, audio) data to multiple receivers located in customer premises. However, inside the customer premises, further distribution of this content may be required. For example, distribution of IPTV content from an IPTV gateway device to a set top box (STB), a desktop personal computer (PC), a dual-mode smart phone, a client, a client device, a mobile device, a mobile terminal, an end device, a terminal device, a laptop or any other equivalent device. In many cases this distribution occurs over a wireless network (e.g. IEEE 802.11a/b/g/n) which is lossy in nature. As a result data packets will invariably be lost during transmission over the wireless network. If multicast is used for data transmission over the lossy network, packet losses will be more severe since there is no link layer packet recovery that is used to correct these errors. However, if unicasting is used for transmission over the lossy wireless network, because of the link layer retransmissions, the performance of the network in terms of packet losses improves.
It would be beneficial to convert a multicast session to a unicast session in the above situation. Multicast-to-unicast conversions are fraught with pitfalls including the necessity for increased memory to maintain multiple copies of the data.
The protocols used for multicasting data over IP networks have been enhanced to support reliability. IETF RFC 2887 describes ACK and NACK based mechanisms to enable reliable data transmissions. Other techniques use Forward Error Correcting Codes (FEC) and Automatic Repeat Request (ARQ). Use of an ACK/NACK mechanism requires that the sender of the data be aware of the ACK/NACK mechanism in order to correct for errors. Similarly, for FEC based solutions, the sender needs to create the additional FEC packets in order to enable data packet recovery at the receivers. Yet another mechanism changes the link layer header of multicast packets from multicast IP address to unicast IP address. This scheme does not work in the case where there are one or more routers between the wireless receiver and the link layer multicast-to-unicast converter.